Producing ethylene by methane dehydrogenation is an energy-intensive reaction. Since the reaction is endothermic and reaction temperatures greater than 800° C. are generally required to achieve practical methane conversion levels, a significant amount of heat is required to maintain the reaction. Generating this heat and transferring it to the methane is a significant cost and can introduce inefficiencies into the process. In order to overcome some of these difficulties, there has been considerable effort directed toward methane conversion via catalytic oxidative coupling reactions.
One process for producing ethylene from methane by catalytic oxidative coupling is disclosed in Synthesis of Ethylene via Oxidative Coupling of Methane, G. E. Keller and M. H. Bhasin, Journal of Catalysis 73, 9-19 (1982). Although an appreciable selectivity to ethylene was observed (to a maximum of about 50%), conversion was relatively low. In order to overcome the methane-ethylene separation difficulties resulting from the low methane conversion, technology has been developed for quenching the reaction product downstream of the oxidative coupling reactor, and then separating ethylene from the unreacted methane.
One process, disclosed in Enhanced C2 Yields from Methane Oxidative Coupling by Means of a Separative Chemical Reactor, A. E. Tonkovich, R. W. Carr, R. Aris, Science 262, 221-223, 1993, includes a simulated countercurrent moving-bed chromatographic reactor, and achieves 65% methane conversion and 80% selectivity to C2 hydrocarbons. The reactor is configured in four sections, with each section comprising (i) a catalytic reactor containing Sm2O3 catalyst and (ii) an adsorbent column located downstream of the catalytic reactor. Methane and oxygen react via catalytic oxidative coupling in the reactor at a temperature in the range of about 900° K to 1100° K, and then ethylene is separated from unreacted methane in the sorption column. In order to maintain sufficient selectivity for ethylene sorption, the reactor's product is quenched to a temperature of 373° K in the sorption column. In another process, disclosed in Methane to Ethylene with 85 Percent Yield in a Gas Recycle Electrocatalytic Reactor-Separator, Y. Jiang, I. V. Yentekakis, C. G. Vayenas, Science 264, 1563-1566, 1994, gas recycle is utilized to further increase methane conversion, but an even lower quench temperature (30° C.) is used during ethylene sorption.
Although the disclosed moving-bed and gas-recycle processes improve conversion, the quenching is energy intensive, and further improvements are desired. Further improvements are particularly desired in converting alkanes such as methane into C2+ olefins such as ethylene and propylene, particularly with increasing selectivity to ethylene production.
Separation of the C2 components from the reaction mixture produced by the methane dehydrogenation process is also energy intensive in that cryogenic separation techniques are generally employed. Cryogenic separation involves the fractional distillation of components in a component mixture, in which the component mixture has been cooled to very low temperatures in order to separate the various components. Since methane dehydrogenation processes typically produce reaction mixtures comprising of multiple components such as ethylene, ethane, acetylene, propylene and carbon dioxide, as well as unreacted methane, and these types of components have very low boiling points, cryogenic separation is typically used in order to separate the individual components. Cryogenic separation generally requires refrigeration on the order of −150° C. to −100° C. in order to liquefy certain components, which is necessary in order to separate the various components. Thus, a substantial amount of energy is needed to reach appropriate cryogenic separation conditions.
U.S. Patent Pub. No. 2014/0018589 to Iyer et al. discloses systems and methods for reacting methane in an oxidative coupling of methane (“OCM”) process to yield products comprising hydrocarbon compounds with two or more carbon atoms (also “C2+ compounds”), and separating the products into streams for use in various downstream processes. The product stream is sent to a dryer to remove water, then to a nitrogen removal unit (NRU) to selectively adsorb nitrogen from the hydrocarbons by pressure swing adsorption. The remaining hydrocarbon product is then compressed and sent to a refrigeration unit to separate the C3+, ethane, ethylene and methane components. The hydrocarbon products can be separated with less use of refrigeration, e.g., by using relatively smaller refrigeration units, provided water, CO2, and nitrogen are removed from the reaction products. But even with these improvements, cryogenic (i.e., very low temperature) separation is still required to separate the particularly desired C2 components and produce a C2 rich product. It is desired to lessen or substantially eliminate the need for cryogenic separation, to achieve a more energy-efficient separation of the reaction mixture's C2 components. In particular, further improvements are desired to achieve a substantially non-cryogenic separation of the C2 components from the reaction mixtures.